How to set up for an effective focus pull?

[daamon]

Hi all,

I know what focus pulling is and the principals behind it - i.e. two or more subjects at different distances from the camera and the focus is manually adjusted to bring each one into and then out of focus in turn.

My question is about how best to set the camera up so that I can get a crisp focus on one subject with the other very obviously out of focus.

1. I've tried it with the cam very close to the near subject and the background subject a few feet away.

2. I've tried it with the cam a few feet away from the near subject, with the background subject further back still.

3. Same as 2, but with a close up zoom on the near subject.

I get reasonable results, but nothing to the extent of really in focus vs very clearly totally out of focus.

I know that this depends on having a decent depth of field which in turn is closely related to the iris aperture width (the smaller the better), thus both subjects need to be well lit. I guess it's more the positioning of the cam and the use (or not) of zoom that I'm asking about.

Cheers.

[Cornucopia]

Depth of field is determined by:

1. Aperture (smaller = more, but less light available)
2. Focal length (shorter = more, but wider angle)<--Only indirectly (influenced by #1 & #3 and distance)
3. Imager size (smaller = more, but lower resolution?)

You could always get an ND filter and or shortening the electronic shutter speed, which will mean opening up the Iris (aperture) to maintain good exposure, thus making your depth of field smaller.
You could try a "depth-of-field" converter, which optically cheats the lens to thinking it's using a larger imager, thus less DOF. This isn't without problems, as everything is usually reversed/upsidedown and your effective focal length is changed. Check out MediaChance's do-it-yourself DOF kit...http://www.mediachance.com/dvdlab/dof/index.htm

The other thing you mentioned includes focal length, but you need to reposition yourself if you expect to account for changes in angle width (aka Field-of-View), and this will then have consequences with foreshortening/geometric exaggeration.

You may need to do a combination of all these to get what you're looking for...

Good luck,
Scott

[daamon]

Hi Cornucopia,

Thanks for the info. Appreciated.

I have a Canon XM2 that has an inbuilt ND filter, and allows the manual setting of the shutter speed (among other things) - so both are good things for shortening the DOF and thus facilitating crisper focus pulls (right?).

The kind of subjects I typically use the effect on are static in both the foreground and background. E.g. a flower close-up with a church entrance in the background.

Thanks for the help - I think I've got it, just gotta try it out!

[racer-x]

You want to shoot in manual settings. Set the Arpiture to lowest numerical setting like 1.6 ect. Set the zoom to highest optical setting like 12x ect. If you have a macro Mode, you can try that also.

If it is a static shot, you can take one shot of the near object in focus, then take a shot of the far object in focus and use crossfade on the Timeline. You can also do this using a Still camera.

[daamon]

Here's an example of the kind of thing I have as subjects and the typical results I've achieved so far: flowers in the foreground, pagoda in the background.

focus_pull.avi - Xvid / 352 x 288 / 516Kb. Source: DV AVI

You can see that, when the flowers are in sharp focus, the pagoda is still recognisable though out of focus. Likewise, when the focal point moves to the pagoda, the flowers aren't too far off of being sharp.

Due to the Xvid codec, lower resolution and low bitrate (for a smaller file to post), the image looks more out of focus than it does on the original DV AVI.

I'm looking to take shots like this where the contrast between in focus and out of focus is more pronounced.

@ racer-x: Thanks for the info. My cam, as far as I know, doesn't have macro mode - I'll check in the manual. Crossfading two shots would work, but I'm a purist and want to do it "properly", so to speak.